Stainless steel resistant to delayed cracking and a method for its production

ABSTRACT

The invention relates to a stainless steel exhibiting transformation-induced plasticity (TRIP) effect resistant to delayed cracking and to the method for producing the stainless steel. The resistance to delayed cracking in the stainless steel is achieved limiting the total hydrogen content of the stainless steel measured by inert gas fusion method below 4 weight ppm, preferably below 3 weight ppm by a heat treatment performed at the temperature range between 100° C. and 700° C. for 0.1-300 hours, preferably at 200-600° C. for 1-100 hours, and more preferably at 250-500° C. for 1-100 hours.

This invention relates to an unstable stainless steel exhibitingtransformation-induced plasticity (TRIP) effect, which is highlyresistant to so-called delayed cracking phenomenon. The stainless steelis an unstable austenitic stainless steel or an unstableaustenitic-ferritic duplex stainless steel. The invention also relatesto a method for producing the stainless steel in order to improve theresistance to delayed cracking, which method leads to a lower hydrogencontent compared to conventional stainless steel production method.

Low-alloyed and unstable austenitic and duplex stainless steelsexhibiting so-called transformation induced plasticity (TRIP) effect dueto transformation of unstable austenite phase to strain-induced ε and/orα′-martensite phases during plastic deformation are susceptible todelayed cracking phenomenon, sometimes also called as season cracking.Delayed cracking manifests itself by cracking of formed metal partsimmediately after the forming process or after a certain period of timeafter the forming. According to the prior art, sensitivity of the εand/or α′-martensite, most probably that of α′-martensite phase, to thehydrogen inherently contained by the steel is the main cause of thecracking phenomenon.

Delayed cracking is a serious problem, as it limits the use of unstablestainless steel in a wide range of application areas, where severeforming operations are needed. Particularly problematic are formingmethods which result in high residual tensile stresses in the formedcomponent. Deep drawing is an example of such forming process.Therefore, it is of high importance to be able to control and avoid thedelayed cracking phenomenon.

In particular, steels having a low nickel content to improve the costefficiency of the steel are highly susceptible to delayed cracking.Stainless steels susceptible to the delayed cracking phenomenon cover awide range of chemical compositions, contents of main alloying elementsranging typically as follows: chromium 15-20 weight %, nickel content0-8 weight %, manganese content 0-10 weight %, nitrogen content 0-0.3weight %, carbon content 0-0.1 weight %, copper content 0-3 weight %.Examples of such commercially available steels are, for instance, steelgrades AISI 301, AISI 301 LN, AISI 201, AISI 201 LN and AISI 204Cu.

New low-nickel austenitic stainless steels have also been disclosed inseveral patent documents. Examples of these patent documents include,for instance, EP 0694626, U.S. Pat. No. 3,893,850 and EP 0593158.However, none of these documents reveal means to avoid the delayedcracking.

Delayed cracking is related to the formation and presence ofstrain-induced martensite phases during the plastic deformation. Thus,delayed cracking can be prevented by careful fine tuning of the chemicalcomposition of the stainless steel so that the formation of martensiteduring plastic deformation is prevented, i.e., the stainless steel ismade stable against the martensite formation. Such an austeniticstainless steel is described in WO publication 2011/138503. However, theproblem of such an approach is that the mechanical properties of thesteel must be compromised. Formation of strain-induced martensite phasesduring deformation enhances the tensile strength and elongation due tothe TRIP (TRansformation Induced Plasticity) effect, which results in asuperior combination of strength and elongation compared to a stablestainless steel. Such property combination is particularly useful inlightweight structures where high crash resistance and energy absorptioncapacity is required. Stainless steels according to WO publication2011/138503 are stable and do not exhibit the TRIP effect. Thus, theircombination of tensile strength and elongation is inferior to stainlesssteels exhibiting strain-induced martensite formation and the TRIPeffect.

According to the prior art, low-nickel austenitic stainless steelsexhibiting the TRIP effect and thus desirable mechanical propertieswithout the susceptibility to delayed cracking cannot be produced.

Conventional austenitic-ferritic duplex stainless steels consist offerrite phase and stable austenite phase, which does not transform tomartensite during plastic deformation. Thus, conventionalaustenitic-ferritic stainless steels are not susceptible to delayedcracking phenomenon. However, a recently developed novelaustenitic-ferritic duplex steel contains unstable austenite phasetransforming to strain-induced martensite phase during plasticdeformation, i.e. the steel exhibits the TRIP effect. This feature makesthe combination of strength and elongation of the novelaustenitic-ferritic stainless steel superior compared to conventionalaustenitic-ferritic stainless steels. However, due to the TRIP effect,the steel is also susceptible to delayed cracking phenomenon, whichlimits its applicability. The steel is described in the publications WO2012/143610 and WO 2011/135170, but in these publications there is nomeans to avoid delayed cracking in such austenitic-ferritic stainlesssteel exhibiting the TRIP effect.

It is known that the susceptibility of austenitic stainless steel tohydrogen embrittlement can be reduced by controlling the hydrogencontent of the steels. However, no means for avoiding delayed crackingof unstable austenitic or austenitic-ferritic duplex stainless steelexhibiting the TRIP effect have been disclosed.

The EP patent application 2 108 710 discloses a method to removehydrogen from an austenitic stainless steel. However, this EP patentapplication does cover only austenitic stainless steels containing morethan 8% nickel, i.e., does not consider low-nickel austenitic stainlesssteels or austenitic-ferritic stainless steels. The steels of this EPpatent application do not exhibit TRIP effect enhancing mechanicalproperties. Furthermore, the steels of this EP patent application areknown to be practically stable against the strain-induced martensitetransformation and thus resistant to delayed cracking. This EP patentapplication does not provide means for avoiding delayed crackingphenomenon in unstable austenitic stainless steels, but focuses onreduction of fatigue crack growth rate by control of hydrogen content.Furthermore, the method of this EP patent application aims to reduce thehydrogen content to unnecessarily low level, and suggests that the heattreatment should be carried out in a very low pressure (vacuum), whichis not practical in industrial scale production.

The JP patent applications 1998-121208 and 2005-298932 relate to anunstable austenitic stainless steel wire. In these JP patentapplications in order to avoid so-called longitudinal crackingphenomenon in drawn wire, a heat treatment method to reduce hydrogencontent of the steel is proposed. However, these JP patent publicationsdo not consider delayed cracking phenomenon in flat stainless steelproducts, and do not provide means to avoid the delayed crackingphenomenon in austenitic or austenitic-ferritic stainless steels.

The object of the present invention is to prevent drawbacks of the priorart and to produce a stainless steel exhibiting transformation-inducedplasticity (TRIP) effect with improved resistance to delayed cracking bylimiting the hydrogen content, which stainless steel is an unstableaustenitic stainless steel or an unstable austenitic-ferritic duplexstainless steel. The present invention relates also to a productionmethod of such a stainless steel. The essential features of the presentinvention are enlisted in the appended claims.

The present invention relates to a stainless steel, an unstablelow-nickel austenitic stainless steel or an unstable austenitic-ferriticduplex stainless steel particularly in the form of a flat product, whichstainless steel exhibits formation on strain-induced martensite duringdeformation (TRIP effect) enhancing their mechanical properties, butwhich is resistant to delayed cracking. The resistance to delayedcracking is achieved limiting the hydrogen content of the steel below 4weight ppm (parts per million), preferably below 3 weight ppm measuredby inert gas fusion method. The steel according to the inventioncombines desired features, such as low nickel content, excellentcombination of strength and elongation due to formation ofstrain-induced martensite phase during plastic deformation (TRIPeffect), and low susceptibility to the delayed cracking phenomenon. Inthe method of the invention the material is heat treated at thetemperature range of 100-700° C. to control the hydrogen content of thestainless steel and to improve the resistance of the stainless steel todelayed cracking. The improved resistance for delayed cracking in thestainless steel of the invention is shown by deep drawing, and in thisdeep drawing a drawing ratio up to 2.0 or even higher is achievedwithout occurrence of delayed cracking.

In accordance with one embodiment the stainless steel of the inventionis an austenitic stainless steel containing in weight % 0-0.15% C, 0-3%Si, 0-15% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.5% N, 0-0.5% Nb,0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impuritiesincluding hydrogen.

In accordance with another embodiment the stainless steel of theinvention is a duplex austenitic ferritic stainless steel whichmicrostructure contains 10-95%, preferably 30-90% ferrite phase andwhich contains in weight % 0-0.10% C, 0-2% Si, 0-10% Mn, 10-30% Cr, 0-8%Ni, 0-3% Mo, 0-3% Cu, 0-0.4% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, thebalance of Fe and inevitable impurities including hydrogen.

The stainless steel exhibiting transformation-induced plasticity (TRIP)effect according to the invention is advantageously in the form of aflat product such as a plate, a sheet, a strip, a coil.

The stainless steel and its production method according to the inventionis based on the reduction and the control of the hydrogen content of thestainless steel by a heat treatment. The heat treatment shall be carriedout at a temperature so that the microstructure and other properties ofthe stainless steel are not significantly affected, but to enablesufficient rapid effusion of hydrogen from the material. The duration ofthe heat treatment is determined by reaching sufficient reduction in thehydrogen content, so that a desired improvement in cracking resistanceis achieved.

According to invention the resistance to delayed cracking is improved bya heat treatment performed at temperature ranging between 100° C. and700° C. for 0.1-300 hours, preferably at 200-600° C. for 1-100 hours,and more preferably at 250-500 ° C. for 1-100 hours.

The stainless steel according to the invention is produced via theconventional stainless steel process route including among othersmelting in electric arc furnace, AOD (Argon Oxygen Decarburization)converter and ladle treatments, continuous casting, hot rolling, coldrolling, annealing and pickling. After the conventional processing ofthe stainless steel to a cold rolled flat product, the material is heattreated according to the invention to control the hydrogen content ofthe stainless steel and improve the resistance of the steel to delayedcracking. According to the invention, this heat treatment can be carriedout in air atmosphere, in atmosphere containing at least partlyprotective gas or in vacuum. Either continuous or batch process may beused. The stainless steel according to the invention may also bestrengthened by temper rolling, i.e. by subjecting the steel to desiredcold rolling reduction of 0.1-60% either before or after the heattreatment according to the invention.

The present invention is described in more details referring to thefollowing drawings, in which

FIG. 1 shows cup samples deep drawn to drawing ratio of 2.12 fromaustenitic stainless steel of the invention in cold-rolled, annealed andpickled condition (as supplied) and after heat treatment of cold-rolled,annealed and pickled material at 400° C. for 3 (400° C./3 h), 24 (400°C./24 h) and 72 hours (400° C./72 h) in air atmosphere,

FIG. 2 shows cup samples deep drawn to drawing ratio of 2.0 fromaustenitic 5 stainless steel of the invention in cold-rolled, annealedand pickled condition (as supplied) and after heat treatment ofcold-rolled, annealed and pickled material at 400° C. for 3 (400° C./3h), 24 (400° C./24 h), and 72 hours (400° C./72 h) in air atmosphere,

FIG. 3 shows cup samples deep drawn from austenitic-ferritic duplexstainless steel of the invention in cold-rolled, annealed and pickledcondition (as supplied) and after heat treatment of cold-rolled,annealed and pickled material at 300° C. for 24 (300° C./24 h) and 72(300° C./72 h), hours and at 400° C. for 24 (400° C./24 h) and 72 hours(400° C./72 h) in air atmosphere,

FIG. 4 shows the influence of heat treatment at 400° C. on totalhydrogen content of austenitic stainless steel of the invention measuredby inert gas fusion method with Leco TCH 600 analyser, and

FIG. 5 shows the influence of heat treatment at 300° C. and 400° C. ontotal hydrogen content of the austenitic-ferritic duplex stainless steelof the invention measured by inert gas fusion method with Leco TCH 600analyser.

The stainless steel of the invention was tested by deep drawing, and adrawing ratio up to 2.0 or even higher is achieved without occurrence ofdelayed cracking. The drawing ratio is defined as the ratio of thediameters of a circular blank having a varying diameter and a punch witha constant diameter used in the deep drawing operation.

FIG. 1 shows the effect of heat treatment at 400° C. on delayed crackingof austenitic stainless steel of the invention containing 17% chromium,4% nickel and 7% manganese as the main alloying elements and deep drawnto drawing ratio of 2.12. The as-supplied steel was in cold-rolled,annealed and pickled condition and 0.8 mm thick. The results show thatthe as-supplied material was susceptible to the cracking, whereas theheat treated steel was completely immune to the cracking.

FIG. 2 shows the effect of the heat treatment at 400° C. on delayedcracking of austenitic stainless steel of the invention containing 15%chromium, 1% nickel, 9% manganese and 2% copper as the main alloyingelements and deep drawn to drawing ratio of 2.0. The as-supplied steelwas in cold-rolled, annealed and pickled condition and 1.0 mm thick. Theresults show that the extent of cracking was substantially reduced bythe heat treatment in this 1% nickel containing austenitic steel, whichis inherently very prone to delayed cracking. Although the crackingcould not be fully avoided, the substantially reduced number of cracksin the very severe cup forming operation indicates much improvedmaterial performance in practical applications.

FIG. 3 shows the effect of the heat treatment at 300 and 400° C. ondelayed cracking of an unstable austenitic-ferritic duplex stainlesssteel of the invention containing 20% chromium, 1% nickel, 3% manganeseand 0.2% nitrogen as the main alloying elements and exhibiting TRIPeffect deep drawn to drawing ratio of 2.12. The as-supplied stainlesssteel was in cold-rolled, annealed and pickled condition and 1.0 mmthick. According to the results, the as-supplied material wassusceptible to delayed cracking, whereas the cracking was fully avoidedin the material heat treated according to the invention.

FIG. 4 shows the influence of heat treatment at 400° C. on totalhydrogen content of austenitic stainless steel of the invention. FIG. 5shows the influence of heat treatment at 300° C. and 400° C. on totalhydrogen content of the austenitic-ferritic duplex stainless steel ofthe invention. According to the results, the hydrogen content of thematerial was clearly reduced by the heat treatment. However, it isapparent that still moderate hydrogen content, around 2 ppm, can beaccepted. This is an important because reaching very low hydrogencontent may require impractically long heat treatment time and use ofvacuum, which both are not desired from the industrial applicabilityviewpoint. When assessing the influence of hydrogen content on delayedcracking phenomenon it is important to bear in mind that the accuratemeasurement of low hydrogen contents is rather difficult. There isscatter between individual measurements and measurements performed withdifferent instruments can show inconsistency.

According to the invention the delayed cracking resistance of austeniticstainless steels or austenitic-ferritic stainless steel exhibiting theTRIP effect is improved by reducing the hydrogen content to level ofabout 2 ppm by performing a proper heat treatment for the material. Thetemperature and the time for the heat treatment are selected so thatenough hydrogen is effused from the material. At temperatures lower than300° C. too slow hydrogen diffusion would lead to impractically longholding times. At temperatures higher than 400° C. there is a risk ofprecipitation of carbides and nitrides and other undesired changes inthe microstructure of the steel. The heat treatment according to theinvention was carried out in air atmosphere, which at the studiedtemperatures leads to oxidation of the surfaces. This can be avoided bycarrying out annealing in a protective atmosphere, like in nitrogen orargon, or most preferably, in vacuum. Minimization of hydrogen partialpressure of the atmosphere will also facilitate effusion of the hydrogenfrom the material and enable reaching lower hydrogen contents.

In industrial scale the heat treatment according to the invention can berealized by utilizing a batch furnace, such as a bell furnace and thegas atmosphere which contains at least partly inert protecting gas suchas nitrogen or argon. Also the utilization of a continuous annealingline is possible if the atmosphere, temperature and holding time areproperly chosen to enable sufficient removal of hydrogen from thematerial.

1. Stainless steel exhibiting transformation-induced plasticity (TRIP)effect resistant to delayed cracking, characterized in that theresistance to delayed cracking for a flat stainless steel product isachieved limiting the total hydrogen content of the steel measured byinert gas fusion method below 4 weight ppm, by a heat treatmentperformed at the temperature range between 100° C. and 700° C. for0.1-300 hours, preferably at 200-600° C. for 1-100 hours.
 2. Stainlesssteel exhibiting transformation-induced plasticity (TRIP) effectaccording to the claim 1, characterized in that the stainless steel isan austenitic stainless steel containing in weight % 0-0.15% C, 0-3% Si,0-15% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.5% N, 0-0.5% Nb,0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impuritiesincluding hydrogen.
 3. Stainless steel exhibiting transformation-inducedplasticity (TRIP) effect according to the claim 1, characterized in thatthe stainless stainless steel is a duplex austenitic-ferritic stainlesssteel whose microstructure contains 10-95% ferrite phase and whichcontains in weight % 0-0.10% C, 0-2% Si, 0-10% Mn, 10-30% Cr, 0-8% Ni,0-3% Mo, 0-3% Cu, 0-0.4% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balanceof Fe and inevitable impurities including hydrogen.
 4. Stainless steelexhibiting transformation-induced plasticity (TRIP) effect according toclaim 1, characterized in that the stainless steel is in the form of aflat product such as a plate, a sheet, a strip, a coil.
 5. Stainlesssteel exhibiting transformation-induced plasticity (TRIP) effectaccording to claim 1, characterized in that for the stainless steelafter deep drawing a drawing ratio up to 2.0 or even higher is achievedwithout occurrence of delayed cracking.
 6. Method for producing astainless steel exhibiting transformation-induced plasticity (TRIP)effect and resistant to delayed cracking, characterized in that for theresistance to delayed cracking the steel is heat treated at thetemperature range between 100C and 700° C. for 0.1-300 hours.
 7. Methodaccording to the claim 6, characterized in that the steel is heattreated in a batch furnace, to reduce hydrogen content of the steel andimprove the resistance to delayed cracking.
 8. Method according to theclaim 6, characterized in that the steel is heat treated in a continuousannealing line to reduce hydrogen content and improve the resistance todelayed cracking.
 9. Method according to the claim 6, characterized inthat the heat treatment is performed in atmosphere containing at leastpartly protective gas to enhance effusion of hydrogen from the steel.10. Method according to the claim 6, characterized in that the heattreatment is performed in vacuum to enhance effusion of hydrogen fromthe steel.
 11. Method according to the claim 6, characterized in thatthe heat treatment is performed in air atmosphere to enhance effusion ofhydrogen from the steel.
 12. Method according to claim 6, characterizedin that the steel is strengthened by cold-rolling before the heattreatment.
 13. Method according to claim 6, characterized in that thesteel is strengthened by cold-rolling after the heat treatment.